In the absence of a dopant or precursor modification, anatase to rutile transformation in synthetic TiO2 usually occurs at a temperature of 600−700 °C. Conventionally, metal oxide dopants (e.g., Al2O3 and SiO2) are used to tune the anatase to rutile transformation. A simple methodology is reported here to extend the anatase rutile transformation by employing various concentrations of urea. XRD and Raman spectroscopy were used to characterize various phases formed during thermal treatment. A significantly higher anatase phase (97%) has been obtained at 800 °C with use of a 1:1 Ti(OPr)4:urea composition and 11% anatase composition is retained even after calcining the powder at 900 °C. By comparison a sample that has been prepared without urea showed that rutile phases started to form at a temperature as low as 600 °C. The effect of smaller amounts of urea such as 1:0.25 and 1:0.5 Ti(OPr)4:urea has also been studied and compared. The investigation concluded that the stoichiometric modification by urea 1:1 Ti...

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Synthesis of High-Temperature Stable Anatase TiO2 Photocatalyst

A liquid-phase redox process has been designed to prepare monodispersed Co3O4 nanocrystals with particle sizes of 2 nm (spherical), 2.5 nm (cubelike), and 4.7 nm (cubelike). The nanocrystals were characterized by transmission electron microscopy (TEM), high-resolution TEM, X-ray diffraction, X-ray photoelectron spectroscopy, Fourier transform infrared spectroscopy, and thermogravimetric techniques. The nucleation and growth, which were tracked by UV−visible spectroscopy, can be separated by adjusting the solubility of sodium nitrate, and the smallest possible Co3O4 nanocubes, 2.5 nm on a side, were obtained. A solubility-controlled mechanism for the redox reaction is discussed which is critical in avoiding secondary nucleation and interparticle ripening growth of Co3O4 nanocrystals.

Solubility-Controlled Synthesis of High-Quality Co3O4 Nanocrystals

Photocatalytic evolution of hydrogen over mesoporous TiO2 supported NiO photocatalyst prepared by single-step sol–gel process with surfactant template

Nanocrystalline alumina was synthesized by using the sol−gel method with aluminum sec-butoxide as the precursor. FTIR, TG, XRD, and crystalline structure refinement were used to analyze the phase transformations and their crystalline structures. When samples were annealed at 200 °C, they had two nanocrystalline phases γ-boehmite and γ*-boehmite, which had the same crystalline structure but different lattice parameters and defect numbers in the structure. After sample annealing above 400 °C, the γ-boehmite and γ*-boehmite phases were transformed into nanocrystalline γ-Al2O3 and θ-Al2O3. The γ-alumina crystalline structure contained hydroxyls that brought about cationic defects in aluminum sites, mainly in those of octahedral symmetry. In the 27Al NMR spectra of the samples calcined at 400, 600, and 800 °C, a chemical shift around 33 ppm was observed, which occurred between that of aluminum in oxygen octahedra and aluminum in oxygen tetrahedra. Its origin can be explained by assuming the existence the subst...

Aluminum Local Environment and Defects in the Crystalline Structure of Sol−Gel Alumina Catalyst

Of the various forms of titania (anatase, rutile, and brookite), anatase is reported to be the best photocatalyst. The anatase-to-rutile transformation in pure synthetic titania usually occurs at a temperature range of 600−700 °C. High-temperature (≥800 °C) stable anatase titania photocatalyst is required for antibacterial applications in building materials. A simple methodology to extend the anatase phase stability by modifying the titanium isopropoxide precursor with sulfur modification using sulfuric acid is presented. Various TTIP/H2SO4 molar ratios such as 1:1, 1:2, 1:4, 1:8, and 1:16 were prepared, and these samples were characterized by XRD, DSC, Raman spectroscopy, XPS, and BET surface area analysis. Sulfur-modified samples showed extended anatase phase stability up to 900 °C, while the control sample prepared under similar conditions completely converted to rutile at 800 °C. Stoichiometric modification up to 1:4 TTIP/H2SO4 composition (TS4) was found to be most effective in extending the anatase-...

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Improved High-Temperature Stability and Sun-Light-Driven Photocatalytic Activity of Sulfur-Doped Anatase TiO2

Tetragonal Nanophase Stabilization in Nondoped Sol–Gel Zirconia Prepared with Different Hydrolysis Catalysts

In the sol−gel Cu/TiO2 system, nanocrystalline brookite, which was stable even after annealing the sample at 400 °C, was generated in high concentrations. This was achieved with copper chloride as copper precursor. The respective samples had a light absorption band at 562 nm that corresponded to an energy bandgap of 2.30 eV, which is smaller than the values reported for titania. When copper sulfate was the precursor, brookite was absent, but nanocrystalline anatase, the main phase, was stabilized even at 800 °C by sulfate ions. After annealing the samples at low temperatures, copper was well dispersed for 1 wt % copper, but part of it segregated forming copper compounds when the copper concentration was 10 wt %. By annealing the samples at 800 °C, copper oxidized to CuO; except in the 10 wt %-copper sample prepared with copper sulfate, where it formed antlerite.

Effect of Copper Precursor on the Stabilization of Titania Phases, and the Optical Properties of Cu/TiO2 Prepared with the Sol−Gel Technique

Magnesium−alumina mixed oxide catalysts at Mg:Al atomic ratios of 3:1, 2:1.8, and 1:2.8 were synthesized by using the sol−gel technique. Dehydroxylation and phase transformations were studied with ...

A. Morales

Papers

Tetragonal Nanophase Stabilization in Nondoped Sol–Gel Zirconia Prepared with Different Hydrolysis Catalysts

Effect of Copper Precursor on the Stabilization of Titania Phases, and the Optical Properties of Cu/TiO2 Prepared with the Sol−Gel Technique

Cationic and Anionic Vacancies in the Crystalline Phases of Sol−Gel Magnesia−Alumina Catalysts